The present invention relates to an aluminum alloy for magnetic disc substrate and, in particular, to the aluminum alloy excellent in the platability, which improves the adhesion of electroless plating in the priming plating, smoothness of plated surface, etc.
For the magnetic disc used for the recording device of electronic computer, such one as covered the surface of substrate comprising aluminum alloy with magnetic substance is generally used. Such magnetic disc is produced in a way that, after the substrate was machined to a fixed thickness and the surface was polished mirror-like, a mixture of powder of magnetic substance with powder or resin is coated, and then heat treatment is carried out to form the film of magnetic substance.
Recently, large capacity and high density have become to be required for the magnetic disc. Thus, the magnetic domain per bit of magnetic disc is being made more and more minute and, at the same time, the clearance between magnetic head and magnetic disc has also become necessary to be decreased. As a result, requirements to make thinner and to improve the wear resistance have arisen also for the film of magnetic substance. For this reason, a magnetic disc is proposed, wherein, after the substrate was machined to a fixed thickness and the surface was processed mirror-like, a hard nonmagnetic metal, for example, Ni-P is plated electrolessly as a primer for covering with magnetic substance, and than a magnetic substance, for example, Co-Ni-P alloy is coated by sputtering or plating.
For such magnetic disc substrate, following characteristics are requested.
(1) To be a type not necessitating the heat treatment and to have sufficient strength to endure the high-speed rotation at the time of various processing and use. PA1 (2) To be light-weight, to be capable of obtaining good mirror-like surface by polishing and not to appear the surface defects such as pits etc. PA1 (3) To be excellent in the adhesion of electroless plating being the priming treatment and the smoothness of surface and not to appear the defects such as pits etc. also after plating.
As a substrate for the magnetic disc satisfying such characteristics, JIS A5086 alloy (Mg 3.5-4.5 wt. %, Fe.ltoreq.0.50 wt. %, Si.ltoreq.0.40 wt. %, Mn 0.20-0.7 wt. %, Cr 0.05-0.25 wt. %, Cu .ltoreq.0.10 wt. %, Ti.ltoreq.0.15 wt. %, Zn.ltoreq.0.25 wt. % and the remainder A1) or an alloy in which the intermetallic compounds produced in the matrix are made smaller by regulating Fe, Si, etc. being impurities in JIS A5086 alloy is used.
With the substrate comprising said JIS A5086 alloy, however, because of the poor adhesion of electroless plating being the priming treatment for covering with magnetic substance, there has been a problem that the covering of electroless plating is sometimes peeled off in the coating process with magnetic substance or during use. Moreover, the surface smoothness after electroless plating cannot also be said to be sufficient. Namely, the intermetallic compounds drop out at the time of zincate treatment to produce pits. These pits disappear in many cases by subsequent polishing if the thickness of electroless plating is as thick as 20 .mu.m, but, because of the recent tendency to make the plating thickness thinner (for example, 17 .mu.m), such a case that the pits are left behind also after polishing subsequent to plating has been caused. Moreover, the plate of aluminum alloy is punched out to a fixed size and then submitted to cutting or grinding polishing. At this time, the intermetallic compounds drop out in some cases to become pit defect. As described, for the improvement in the palatability of magnetic disc, it has been earnestly desired to decrease mainly the number of the intermetallic compounds of aluminum alloy for the substrate and also to make the size smaller. Various measures have been taken for this, but sufficient results are not necessarily achieved.
Furthermore, the corrosion resistance of the magnetic disc substrate is also one of the important characteristics, but this could not be said to be sufficient hitherto. As a result of various investigations by the inventors, it has become clear that, as the causes to lower said corrosion resistance, minute defects in the film of plating also affect significantly together with the corrosion resistance of Al substrate.
In consequence of extensive investigations in view of said problems, ti has been known that, for the adhesion of the electroless plating with Ni-P alloy and the smoothness of the plated surface, it is necessary to adhere the zincate film of pretreatment thinly, uniformly and densely, and further that the adhesion of the plated film with Ni-P alloy is affected not only by the adhering situation of zincate film but also by the diameter of crystal grains in the material and the trace elements to be added in the matrix, that is, if the crystal grains are minute, the adhesion is enhanced, and the adhesion and the minute defects in the plated film are improved also by controlling the type and quantity of trace elements to be added. After further investigation, the aluminum alloy for the magnetic disc substrate has been developed according to the invention, wherein, by combining said effects if need by, the interactions therebetween can be obtained and, as a result, the adhesion of electroless plating being the priming plating is excellent, yet plated surface is smooth, there are no surface defects, and the corrosion resistance of plated substrate is also excellent.